This invention relates to techniques and apparatus for determining characteristics of earth formations surrounding a borehole and, more particularly, to nuclear magnetic resonance borehole logging that utilizes pulse sequences which improve performance.
General background of nuclear magnetic resonance (NMR) well logging is set forth, for example, in U.S. Pat. No. 5,023,551. Briefly, in NMR operation the spins of nuclei polarize along an externally applied static magnetic field, assumed to be in the z direction. The vector sum of the magnetic moment from individual nuclei is a macroscopic magnetic dipole called the magnetization, M0. The magnetization is normally aligned with the static magnetic field, but the equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g. an RF pulse generated by an RF antenna), which rotates the magnetization away from the static field direction. The length of the RF pulse can be adjusted to achieve a prescribed rotation angle, such as 90 degrees, 180 degrees, etc. After rotating, two things occur simultaneously. First, the spins precess around the static field at the Larmor frequency, given by xcfx890=xcex3B0, where B0 is the strength of the static field and xcex3 is the gyromagnetic ratio. For hydrogen nuclei, xcex3/2xcfx80=4258 Hz/Gauss, so, for example, for a static field of 235 Gauss, the frequency of precession would be 1 MHz. Second, the spins return to the equilibrium direction according to a decay time T1, the spin lattice relaxation time. Also associated with the magnetization is a second relaxation called the spin-spin relaxation with a decay time T2.
A widely used technique for acquiring NMR data both in the laboratory and in well logging, uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. As is well known, after a wait time that precedes each pulse sequence, known as polarization time, a ninety degree pulse rotates the magnetization to the x-y plane. The spins begin to precess around B0 and dephase. After a certain time delay, a one hundred eighty degree pulse is applied to cause the spins which are dephasing in the transverse plane to refocus. Refocusing leads to an echo that is detected by the NMR instrument. By repeated application of one hundred eighty degree pulses, a series of xe2x80x9cspin echoesxe2x80x9d appear, and the train of echoes is measured and processed.
It has been recognized that xe2x80x9cringingxe2x80x9d is a problem encountered when using pulsed nuclear magnetic resonance techniques. There are two types of spurious ringing in pulsed NMR. The first type is electronic ringing which arises from the transient effects of a resonance electronic circuit, and is determined by the characteristics of the resonance circuit. The electronic ringing can be substantially reduced using time-controlled hardware such as Q-switching approaches. The second type of spurious ringing arises from exciting the acoustic resonances in or around the RF antenna structure. [See A. A. V. Gibson and R. E. Raab, xe2x80x9cProton NMR and piezoelectricity in tetramethylammonium chloride,xe2x80x9d J. Chem. Phys. 57, 4688-4693, (1972); M. L. Buess, and G. L. Peterson, xe2x80x9cAcoustic ringing effects in pulsed magnetic resonance probes,xe2x80x9d Rev. Sci. Instrum., 49, 1151-1155, (1978); E. Fukushima, and S. B. W. Roeder, xe2x80x9cSpurious ringing in pulse NMR,xe2x80x9d J. Mag. Resonance, 33, 199-203, (1979); and R. L. Kleinberg, A. Sezginer, D. D. Griffin, and M. Fukuhara, xe2x80x9cNovel NMR Apparatus for Investigating an External Sample,xe2x80x9d J. Mag. Res., 97, 466-485, (1992).] This is magnetoacoustic ringing, and can last up to several milliseconds. It appears whenever the frequency of the applied RF current matches at least one of acoustic resonance modes of the RF antenna or its surrounding structure. Both types of ringing are phase coherent with the applied RF pulse and therefore can not be canceled, unlike incoherent noise, by stacking repeated measurements. [Techniques for dealing with the problems of ringing in laboratory equipment are disclosed in U.S. Pat. No. 4,438,400 and in the following publications: I. P. Gerothanassis, xe2x80x9cMethods Of Avoiding The Effects Of Acoustic Ringing In Pulsed Fourier Transform Nuclear Magnetic Resonance Spectroscopyxe2x80x9d, Progress in NMR Spectroscopy, Vol. 19, pp. 276-329, 1987 (see Section 9.3 and see Note Added In Proof with regard to sequences of interest as used in laboratory spectrometry with single echo sequences); and S. Zhang, X. Wu, and M. Mehring, xe2x80x9cElimination Of Ringing Effects In Multiple-Pulse Sequencesxe2x80x9d, Chemical Physics Letters, Vol. 173, No. 5.6, pp. 481-484, 1990.]
The amplitude of the ringing signal can be large enough to saturate the receiver circuitry, making its response to the CPMG echo signal nonlinear. Magnetoacoustic ringing can be reduced by selecting proper material for the RF antenna and its surrounding structure (see, for example, U.S. Pat. No. 5,153,514), but it is very difficult to completely eliminate acoustic ringing by mechanical methods alone, particularly in well logging equipment that has design constraints relating to its adaptability for the borehole environment. This ringing can be a major obstacle for measuring parameters such as total porosity in magnetic resonance logging.
It is among the objects of the present invention to provide a technique and apparatus for substantially eliminating the effects of phase coherent acoustic ringing in nuclear magnetic resonance well logging.
In accordance with a form of the method of the invention, there is disclosed a technique for obtaining nuclear magnetic resonance measurements from formations surrounding an earth borehole, comprising the following steps: providing a logging device that is moveable through the borehole and through formations in which a static magnetic field is present; producing, from the logging device, a series of cycles of pulse sequences in the formations, each of the pulse sequences including an RF excitation pulse and several RF refocusing pulses; receiving, at the logging device, spin echoes from the formations to produce spin echo signals that may include spurious ringing signals from the excitation and refocusing pulses; and combining spin echo signals from corresponding spin echoes of each of the cycles of pulse sequences to obtain combined spin echo signals in which spurious ringing from the excitation pulses and refocusing pulses of the pulse sequences is substantially cancelled. The static magnetic field can be earth""s magnetic field or a static magnetic field produced at the logging device.
In a preferred embodiment of the invention, the steps of producing cycles of pulse sequences and combining spin echo signals include manipulating the polarities of the excitation and refocusing pulses to obtain the substantial cancellation of the spurious ringing from the excitation and refocusing pulses.
Also in a preferred embodiment of the invention, the series of cycles of pulse sequences comprises four cycles of pulse sequences. In a form of this embodiment, the step of combining spin echo signals from corresponding spin echoes of each of the cycles of pulse sequences to obtain combined spin echo signals comprises combining corresponding spin echo signals from two of cycles and subtracting the spin echo signals from the other two of the cycles. Also in this form of the embodiment, all the spin echo signals of two of the four cycles have a polarity that is opposite to that of all the spin echo signals of the other two of the four cycles.
In a further embodiment of the invention, the step of producing a series of cycles of pulse sequences in the formations further includes producing an RF inverting pulse in some of the cycles of pulse sequences, and the step of receiving spin echoes from the formations to produce spin echo signals includes receiving spin echoes to produce spin echo signals that may includes spurious ringing signals from the inverting pulses, and the combining step includes combining spin echoes of each of the cycles of pulse sequences to obtain combined spin echoes in which spurious ringing from the inverting pulses of the pulse sequences is also substantially cancelled. In a form of this embodiment, the inverting pulses are implemented before the excitation pulses of their respective cycles. The excitation and inverting pulses can be combined into a single pulse in each of a plurality of the pulse sequences.
In one preferred form of the invention, the excitation pulses are 90 degree pulses and the refocusing and inverting pulses are 180 degree pulses.
In an embodiment of the invention, the four sequences are constructed using a set of specific construction rules (detailed hereinbelow) that define the phase relationships of the RF pulses used in the sequences. One example of a four phase cycle that satisfies these rules, is
x[y]n+1, x[y]n+1, xx[y]n, xx[y]n, 
where the first letter represents the rf carrier phase of the RF excitation pulse and subsequent letters represent the rf carrier phases of the RF refocusing pulses (x=0 degrees, y=90 degrees, x=180 degrees, y=270 degrees). All phases are measured relative to an (arbitrarily chosen) reference phase. Examples of other possible forms of this embodiment include:
x[y]n+1, x[y]n+1, xx[y]n, xx[y]n 
x[y]n+1, x[y]n+1, xx[y]n, xx[y]n 
x[y]n+1, x[y]n+1, xx[y]n, xx[y]n 
x[y]n+1, x[y]n+1, xx[y]n, xx[y]n 
x[y]n+1, x[y]n+1, xx[y]n, xx[y]n 
x[y]n+1, x[y]n+1, xx[y]n, xx[y]n 
xe2x80x83x[y]n+1, x[y]n+1, xx[y]n, xx[y]n 
x[y]n+1, x[y]n+1, xx[y]n, xx[y]n 
x[y]n+1, x[y]n+1, xx[y]n, xx[y]n 
and
0xc2x0 45xc2x0 [0xc2x0]n, 180xc2x0 45xc2x0 [0xc2x0]n, 0xc2x0 xe2x88x9245xc2x0 [0xc2x0]n, 180xc2x0 xe2x88x9245xc2x0 [0xc2x0]n 
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.